Relevant mods:
Rustichromia 0.9
Mystical Mechanics 0.18
Mystical Gears 1.1.6
Embers Rekindled 1.14-hotfix

Forge version: 14.23.5.2854

Expected behavior: Mechanical power is combined at the output through the mergebox, allowing multiple windmills to power a single device for use in recipes that require higher mechanical power such as grinding cobblestone to gravel, default ore doubling recipes, and the like

Actual behavior: Mergebox recognizes inputs but does not output any power from the output side. Mergebox functions as intended when used with Embers steam engines. When the game is reloaded (exiting to main menu and starting the world again - exiting the game entirely then going back in will continue the former behavior), however, the mergebox will begin output, but it will only recognize a single input and not combine inputs - Additionally, it will only pass through the lowest power input. Windmills will also no longer fluctuate power with the "wind" after reloading and will maintain a stable power output (usually under 50% of maximum) until the entire axle and gearbox assembly is broken down completely and rebuilt.

https://imgur.com/a/kIFHLi1 Images taken during testing using the mystical gears mechanical dial to measure power

I have since done more testing and research and have now realized this is an intended feature of the mergebox, and apologize for both posting in the wrong issue tracker and not doing further experimentation until earlier today.

My bad!